This invention generally relates to methods and devices for removing plugs from conduits, and is specifically concerned with the removal of nickel alloy plugs from the heat exchanger tubes that are mounted in the tubesheet of a nuclear steam generator.
Tooling systems for removing the plugs from the heat exchanger tubes in a nuclear steam generators are known in the prior art. The use of such devices has become increasingly popular in recent years as new mechanical and heat treating techniques have recently become available for putting such plugged tubes back into service. However, in order to fully appreciate the operation and utility of such prior art plug-pulling tooling systems, a general understanding of the structure and operation of the plugs used in such tubes is necessary.
The plugs that are most frequently used to plug the heat exchanger tubes of nuclear steam generators are formed from a tubular shell of Inconel that is open on its proximal end and closed at its distal end. The interior of the shell contains a further conically shaped expander member. In one of the most commonly used types of plugs, the expander element is shaped like a common cork used to seal a bottle, and is disposed completely within the interior of the shell with its larger circular end in abutment with the inner surface of the closed distal end of the plug shell. The shell walls are not truly tubular, but are slightly tapered from the distal closed end to the proximal open end of the shell. When the cork-shaped expander element is forcefully drawn from the closed end toward the open end of the shell by hydraulic ram, it will radially expand the plug in sealing engagement with the interior wall of the tube by a wedging action. Such a plug design is described in U.S. Pat. No. 4,390,042 invented by Harvey D. Kucherer and assigned to the Westinghouse Electric Corporation. The forceful pulling of the cork-shaped expander element along the longitudinal axis of the shell not only radially expands the wall of the shell outwardly as the expander element is wedgingly drawn towards the proximal end of the shell, but further applies an extruding force to the metallic walls of the shell along the longitudinal axis thereof. The end result is that the lands circumscribing the outer walls of the shell are sealingly engaged against the interior wall of the heat exchanger tube.
Generally, such plugs are used to seal off one or more of the U-shaped heat exchanger tubes contained within a nuclear steam generator when the walls of these tubes become corroded or otherwise damaged beyond repair. This is accomplished by inserting an unexpanded plug into the open end of the tube which is mounted in the tubesheet of the steam generator. A hydraulic ram is then placed over the plug onto the tubesheet and connected to the expander element, which at this point is disposed in abutment with the closed end of the plug shell. The expander element is drawn down by the pull-rod of the ram to the open end of the shell which radially expands it in the manner heretofore described, thereby securing the plug into the open end of the tube. If such corroded or damaged tubes are not plugged, they may crack and allow radioactive water from the primary side of the generator to leak into the non-radioactive water in the secondary side. Such a leakage could, in turn, result in the radioactive contamination of the non-radioactive steam that Westinghouse-type nuclear generators provide to turn the turbines of the electric generators of the plant. Hence the plugging of potentially defective heat exchangers tubes is an important maintenance operation.
Recently, however, new maintenance procedures pioneered by the Westinghouse Electric Corporation have made it possible to repair heat exchanger tubes in nuclear steam generators that were heretofore considered beyond repair. Such techniques include tube expansions to eliminate the annular clearances between the heat exchanger tubes and the tubesheets and support plates in the generator, as well as new stress-relief techniques such as rotopeening and annealing with radiant heaters. As the plugging of a heat exchanger tube diminishes the capacity of a nuclear steam generator to generate power, it has become increasingly popular to remove the plugs that were placed in the open ends of heat exchanger tubes when the repair and maintenance of such tubes becomes possible as a result of the new repair techniques.
Prior art plug-removing devices have generally comprised a push-rod for pushing the expander element back toward the closed end of the shell to relax the shell within the tube, and a pulling fitting concentrically disposed around the push-rod for threadedly engaging and pulling the plug shell from the open end of the tube after the push-rod has moved the expander element upwardly. Such devices are generally powered by a hydraulic ram having two concentrically disposed hydraulic cylinders.
Unfortunately, the use of such prior art plug pulling devices does not always produce perfect results. For example, because of the extruding force that the expander element applies between the lands on the outer shell wall in the interior wall of the tube, some degree of solid phase welding occurs between the shell and the tube. Hence, the hydraulic cylinder connected to the fitting that pulls the plug from the tube may have to apply as much as 14,000 pounds of tensile force before the plug is successfully pulled out. The application of this tensile force in combination with the solid phase welding that has occurred between the plug and the tube may cause a significant amount of galling and scratching to occur within the open end of the tube as a result of the removal operation. Such scratching and galling not only mars the inner surface of the tube, but more importantly, generates stresses in the tube mouth which may give rise to stress corrosion cracking. Other imperfections of such prior art devices include their incompatability for use with commercially-available robotic devices, such as the ROSA developed by the Westinghouse Electric Corporation. The causes of such incompatability include excessive tool weight, and the difficulty of aligning the working ends of these tools with the open end of the tube to be unplugged. Of course, such tools can be operated manually within the channel head of a nuclear steam generator. However, such manual operation may result in the exposure of the operator to potentially harmful radiation.
To avoid the galling and scratching problem, the applicants have developed multi-tool systems that impart a greater degree of relaxation between the outside surface of the plug and the inner surface of the tube before the plug is pulled from the tube. Unfortunately, all of the tooling systems thus far contemplated require the successive use of different tools before the plug is sufficiently relaxed within the tube so as to be removable therefrom without scratching our galling. Hence, the time required to use such systems is substantially greater than the time necessary for a hydraulic ram to simply pull the plug out of the tube mouth. As down-time for such steam generators can cost the utility involved over one-hundred thousand dollars a day in lost revenues, the extra time required for the use of a multiple-tool plug removing system presents an added expense which partially offsets the advantage of scratch-free plug removal.
Clearly, there is a need for a device for removing plugs from the heat exchanger tubes in nuclear steam generators which does not scratch or gall the open ends of the tubes. Ideally, such a device should be remotely operable by means of a commercially available robotic device so as to minimize both the exposure of service personnel to potentially harmful radiation, and the cost of using the device. Finally, it would be desirable if the device were capable of removing plugs at least as quickly as prior art hydraulic ram devices, so that the advantage of scratch-free removal would not be offset by any increase in time required to operate the device.